Disc reproducing system for compensating mechanical imperfections

ABSTRACT

Apparatus for reducing the effects of vertical perturbations of a phonograph disc surface resulting from disc and/or playback system mechanical imperfections. Such imperfections are compensated by closed loop and/or open loop means wherein the deviations of the record path are detected and employed to minimize the effects of the mechanical imperfections.

This application is a continuation-in-part of my copending applicationSer. No. 005,992, filed Jan. 24, 1979, now abandoned, which in turn wasa continuation-in-part of my then copending, now abandoned applicationsSer. No. 859,799, filed Dec. 12, 1977 and Ser. No. 965,423, filed Dec.1, 1978.

BACKGROUND OF THE INVENTION

Commercially manufactured disc phonograph records exhibit variousmechanical imperfections, and further system imperfections result fromthe mechanical means employed to reproduce the record. The presentinvention is concerned with one category of disc reproducing systemimperfections: spurious vertical deviations of the record groove,primarily resulting from record warp and rumble, including recordpressing noise. Such imperfections can cause significant degradation ofthe reproduced signal.

WARP

A general discussion of record warp is included in the followingpublished paper: "Record Warps and System Playback Performance," LarryHapp and Frank Karlov, Journal of the Audio Engineering Society, Vol.24, No. 8, October 1976, pp. 630-638. The authors found warp frequenciesin the range of about 1/2 Hz (the once around frequency at 331/3 rpm) tobeyond 10 Hz, with 95% of the warps below 8 Hz. Peak physical amplitudeheight of the warps was greatest at low frequencies at about 0.025 in.maximum and decreased with increasing frequency.

Various problems are caused by record warp: the tone arm may bounce orsway with respect to the record surface due to the vertical and, to someextent, lateral, forces which result as the stylus attempts to track thevarying record height. This may cause not only variations in trackingforce but bottoming of the cartridge or complete loss of contact betweenthe stylus and groove. Such variations in tracking force from optimumwill often affect the reproduced signal at audible frequencies. Inaddition to causing stylus and arm tracking problems, excessive stylusexcursions result in geometrically related distortions andelectro-mechanical non-linearity of the cartridge. Moreover, subaudiblewarp signals can cause distortion by amplifier overload in electronicsystems passing such low frequencies and, if applied to the spearkersystem, can cause substantial woofer movement that can result inextraneous noises and the distortion of higher frequency audiblesignals, including doppler distortion. Further, the geometricalrelationship of the stylus and record groove is such that a warp resultsin a forward and backward oscillation of the stylus tip over therecorded groove information, which frequency modulates (advances anddelays) the reproduced signal causing "wow". Wow may also result fromvariations in rotational speed as the stylus load on the record groovevaries.

The requirement to track warped phonograph records satisfactorily hasresulted, in prior art systems, in the necessity to considertonearm/cartridge/stylus/record geometry very carefully and to seek thebest combination, usually a comprise, of such factors as stylus and tonearm mass, tone arm damping, stylus compliance and damping, and trackingforce so as to provide a controlled tone arm resonance above thecommonly encountered warp frequencies, yet below the frequency of thelowest recorded groove information. An arm resonance of 10 Hz has beenadvocated by several designers: Keisuka Ikegami and Susumu Hoshimi,"Advance in Turntable and Tone-Arm Design", Journal of the AudioEngineering Society, May 1976, Vol. 24, No. 4, pp. 276-280 and PeterRother, "The Aspects of Low-Inertia Tone-Arm Design", Journal of theAudio Engineering Society, September 1977, Vol. 25, No. 9, pp. 550-559.

Although in principle the proper selection of tone arm and cartridgeparameters may make possible the tracking of warped records, thematching of arms and cartridges is often complicated in practice becauseof the wide variation in tone arms and cartridges available. Further,even at the design stage, the selection of optimum tone arm andcartridge parameters for warp tracking may not be optimum for trackinghigher frequency groove information. Even when the record is properlytracked, the problem of geometric and motor wow from warps stillremains.

Various passive devices for tracking warped records are knonw. Thesedevices typically employ an element riding the record surface and fixedor coupled to the pickup cartridge or the tone arm in the vicinity ofthe cartridge. Such devices include both damped elements and undamped orfixed elements. Exemplary prior art damped element devices are disclosedin U.S. Pat. Nos. 2,572,712 to Fisher (spring loaded plunger), and2,328,862 to Thompson (elastically mounted auxiliary stylus). Fixedelements are disclosed in U.S. Pat. Nos. 3,228,700 to Andrews et al.(felt pad at end of tone arm with cartridge pivoted in tone arm) and3,830,505 to Rabinow (air bearing adjacent cartridge). It is known alsoto employ a dash pot of a brush adjacent the cartridge to damposcillations and assist in tracking warps. Further, proposals for arelatively rigid element coupled in the tone arm to the record surfaceare known. It has also been suggested that the record be clamped orweighted at its periphery and/or center in order to eliminate warp.

An active prior art system for treating record warp is described in thefollowing paper: "Overcoming Record Warps and Low-Frequency TurntableRumble in Phonographs", Kenneth Clunis and Michael J. Kelly, Journal ofthe Audio Engineering Society, July/August 1975, Vol. 23, No. 6, pp.450-458. In this system the cartridge output is used to servo controlthe vertical tone arm position to assist in tracking the record warp.Similar systems are disclosed in U.S. Pat. Nos. 3,623,734 to Sakamoto etal. and 3,830,505 to Rabinow. It is also known to provide a closed looparound the tonearm movements only, in order to improve arm/cartridgedamping. Aspects of the present invention can significantly improve theperformance of these prior art tone arm systems.

RUMBLE

Turntable rumble may result from turntable bearings, motor drivesystems, and environmental vibrations. Considerable efforts are made byturntable manufacturers to eliminate rumble from these sources.

Other turntable related disturbances are caused by acoustic feedback(sonic and infra-sonic) from the loudspeakers, whereby the turntableand/or record may act as a receptor for the vibrations, resulting intonal coloration or even howling. Devices for reducing these effectsinclude a fluid filled turntable mat disclosed in U.S. Pat. No.3,997,174 to Kawashima, and flexible turntable support cups in U.S. Pat.No. 4,054,291 to Maeda, both for providing a conforming damped supportunder warped records.

Notwithstanding these efforts, the main source of low frequencyannoyance is record pressing rumble or mold grain noise from the discitself. The spectrum of record pressing noise is discussed by JohnEargle, "Performance Characteristics of the Commercial Stereo Disc,"Journal of the Audio Engineering Society, August 1969, vol. 17, No. 4,pp 416-422. Mold grain noise may extend generally to several hundred Hz.

Record pressing rumble and turntable rumble are reduced conventionallyby means of high-pass filters in the signal paths or by low frequencychannel blending or cross-coupling schemes. Optimum tone arm/cartridgeresonance characteristics are also useful in reducing low frequencyrumble effects.

SUMMARY OF THE INVENTION

Prior art approaches to dealing with warp are directed primarily to thesymptoms of warp. For example, the passive tone arm to record surfacecontact devices and the closed loop tone arm systems act chiefly asmeans to enhance the ability of the cartridge and tone arm simply totrack warps. Consequently, such approaches may fail to correct othereffects of warp and may degrade tracking ability and signal quality atnon-warp frequencies. With regard to rumble, prior art techniques havebeen directed primarily to electrical filtering or channel blendingrather than to dealing with the rumble mechanism itself.

It is the object of the present invention to reduce the effects of warpand rumble without in any way degrading the bandwidth, channelseparation or other performance characteristics of the signal channelsthemselves.

The present invention is based on observations taken from the situationshown in FIG. 1a and FIG. 1b. FIG. 1a represents a sectional side viewof the top half of a hypothetical record master on which silent grooveshave been recorded. The groove depth "a" is a constant and representsthe instantaneous vertical signal modulation with respect to a perfectreference path or surface. The reference surface may be the flat lacquermaster disc surface, but in accordance with an aspect of the invention,shown in FIG. 2, the reference surface may optionally be defined in thecutting process preferably by a flat secondary cutting stylus followingthe main cutting stylus and arranged to smooth and dimensionally definethe land between the grooves. In some cases, in the frequency range inwhich there is little vertical information recorded on the disc (e.g.below 30 Hz) the groove itself may be used as the reference path.

FIG. 1b represents the situation after making a record pressing of themaster. The vertical groove position is no longer constant but containsirregularities. In the case of warp, these are dimensionally correlatedon the two sides of the record (the thickness remains substantiallyconstant), because they arise simply from thermal and handling relateddistortions during and after removal of the record from the press.Higher frequency mold grain noises, however, are not correlated on thetwo sides of the record, since different dies and stampers are used; thedisc thus contains local variations of thickness. Such imperfections arecaused by the pressure transmittal of dimensional irregularities fromthe back to the front of the stamper during the pressing operation. Theback surface irregularities may include metallic crystals arising fromthe replication process, patterns resulting from grinding operations tosmooth the back surface, dirt and dust trapped between the stamper andthe die of the record press, and surface irregularities of the die.

As the stamper thickness is some 0.007" to 0.010" the rigidity orstiffness of the material will limit the shortest wavelengths which canbe transmitted through localized bending and distortion of the stamper.Thus, such wavelengths might be of the order of 0.020". This results ina highest frequency of mold grain noise at the outer diameter of atwelve inch disc (groove velocity about 20 inches per second) of theorder of 1 kHz.

Further sources of low frequency noise on the record itself may includenon-homogeneity of the pressing material and geometric distortions dueto differential cooling effects resulting from rapid and uneventemperature changes in the die face. Moreover, as discussed previously,noises are also contributed by the reproducing system--namely, turntableand environmental rumble and acoustically transmitted vibrations of theturntable and disc.

Thus, in a conventional reproducing system, the reproduced quantity "b"is obtained, employing the tone arm position as a reference. Thequantity "b" thus includes undesired low frequency noise components.

Closer consideration of this matter shows that the low frequency noisecomponents from all the sources mentioned above are not inextricablymixed with the original signal modulations. Rather, the recorded signalquantity "a" remains intact and unharmed by the pressing and reproducingprocess and by mechanical imperfections in the reproducing system. Thus,the quantity "a" can be recovered if the distorted reference path at thepoint of stylus contact is used as the reference point duringreproduction. Preferably, the reproducer system acts to remove spuriousdeviations of the reference point so that again the disc surface is ineffect flat (i.e., effectively vertically stable) in the vicinity of thestylus. Alternatively, the undulating reference point is used indetermining the true signal quantity "a".

Thus, in accordance with the teachings of the present invention, theproblems of the prior art are solved by a disc record reproducing systemin which mechanical imperfections in the disc or in the system, causingdeviations in the tracked disc path, are compensated by means ofmeasuring an appropriate quantity to produce an error signal and bymeans of utilizing this signal to effect an appropriate correction,mechanically and/or electrically. A sensing means generates a signalresponsive to deviations in a reference path at or in close proximity tothe pickup or stylus and that signal is processed to control the systemin order to minimize the effect on the disc playback signal caused bythe deviations.

In the context of the invention, sensing "in close proximity" meanswithin a small fractional part (e.g., less than about one-tenth) of theshortest wavelength which it is desired to correct. If the mold grainnoises have wavelengths as short as about 0.020 inches, this impliessensing within about 0.002 inches--i.e., on an immediately adjacent landarea. In the simplest form, this would represent the limit of thetechnology of the invention, although with time delay schemes thephysical proximity requirements can be relaxed and the time delayadjusted to effectively provide sensing in close proximity by bringingthe signals in substantial time coincidence. For lower frequency noiseand warp it is, of course, unnecessary to sense in such close proximity.

There are four main embodiments of the invention, which for conveniencemay be referred to as a Vertical Noise Compensator (VNC). Theembodiments may be used separately or in combination.

In all embodiments, the vertical position of an unmodulated portion ofthe record is effectively sensed at or in close proximity to the stylus.The information so derived may be referrred to as reference pathinformation. In some embodiments, the reference path information issensed with respect to the arm or cartridge position; such informationmay be referred to as reference path-arm information. Reference pathinformation or reference path-arm information is preferably obtained viasensing means arranged to sense the land position adjacent the signalgroove and in close proximity to the signal stylus. It is important thatthe vertical sensor responds substantially only to vertical information;in disc recording technology the term "vertical" conventionally has themeaning of perpendicular to the disc surface, or in an axial direction.A less desirable alternative, for the treatment of warp and very lowfrequency rumble effects only, is to sense the groove depth itself.

In the first embodiment, reference path information is obtained and usedin a closed loop servo system including an actuator which moves the discsubstantially vertically, a least in the vicinity of the pickup stylus.Ideally, the result is that at least all vertical disc movement in thevicinity of the pickup stylus is removed, thereby allowing the styluseffectively to track a warp-free and rumble-free record. This embodimentmay be referred to as a disc VNC (or a turntable VNC, inasmuch asvertical actuation of the disc is most readily accomplished viamechanisms associated with the turntable).

A second embodiment, which may be called a tone arm VNC, is animprovement of the closed loop tone arm techniques of the prior art. Inprior art systems, the error signals used include arm-cartridgeresonance components or other misleading information. In the presentinvention, measurements avoiding these defects are made and processed tocontrol the tone arm; namely, reference path information is obtained andemployed to control the tone arm and optionally to perform additionalcorrections via the other embodiments. Low frequency components can beemployed in a turntable VNC for warp compensation and higher frequencycomponents can be used in a cartridge VNC or pre-amp VNC for rumble andmold grain noise reduction.

In a third embodiment, which may be referred to as a cartridge VNC,reference path-arm information is obtained and used to effect correctionwithin or following the cartridge. The reference path-arm information iscancelled either electrically or mechanically from the informationprovided by the signal stylus. The reference path-arm information mayoptionally be brought out of the cartridge in order to performadditional corrections via the other embodiments.

In a fourth embodiment, which may be called a preamp VNC, the referencepath-arm information is obtained and cancelled from the audio outputelectronically in the preamplifier. This arrangement can be used toreduce mold grain noise, rumble and some of the effects of warp.

For optimum mechanical and acoustical performance, it is preferable tocombine the turntable VNC or tone arm VNC methods with the cartridge VNCand/or the pre-amp VNC methods. For example, warp and rumble effects maybe compensated up to, say, 20 Hz using a turntable VNC or a tone armVNC, with frequencies above this being treated by a cartridge VNC orpre-amp VNC.

Compatibility characterizes all the embodiments of the invention.Conventional records may be played on reproducer including theinvention; conversely, records produced with the optional definedreference surface of the invention may be played on conventionalreproducers.

The reduction of noise and tracking problems effected by the inventionmay permit a lower modulation level and a higher groove density to beemployed, leading to longer playing times and/or smaller recorddiameters.

The fact that the invention solves the problem of low frequency noiseleads to the further possibility that higher frequency components of thesignal may be recorded on the disc in electronic noise reduction encodedform, such as by the system known as "Dolby B". This system, whichtreats only those signals above about 1 kHz, produces a compressedsignal which has a proven history of being sufficiently compatible topermit the single inventory manufacturing and distribution of cassettetapes. Such acceptance in the case of encoded discs would be much moredifficult, if not impossible, to achieve on a commercial basis if itwere necessary to treat the low frequency signals as well. The encodeddiscs would, of course, preferably be played back using a noisereduction decoder for reduction of high frequency record pressing noiseand low level ticks and pops.

Thus the present invention can make a significant overall contributionto the current performance and future possibilities of the conventionalanalogue disc record system.

These and other features of the present invention will be appreciated asthe following detailed description is read in connection with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a sectional side view of the top half of a hypotheticalrecord master on which silent grooves have been recorded.

FIG. 1B is a sectional side view of a hypothetical record pressing madefrom the master disc of FIG. 1A.

FIG. 2 is a partly sectional side view of a master disc during thecutting process, using a conventional signal cutting stylus and asecondary reference plane cutting stylus in accordance with one aspectof the present invention.

FIG. 3A is a partially block generalized representation of directreference path information sensing in accordance with one aspect of thepresent invention.

FIG. 3B is a partially block generalized representation of indirectreference path information sensing in accordance with a further aspectof the present invention.

FIG. 4A is a partially cut away perspective view of one type of directreference path information sensing.

FIG. 4B is a partially cut away perspective view of a further type ofdirect reference path information sensing.

FIG. 5 is a partially cut away side view of one type of referencepath-arm sensing.

FIG. 6 is a partially cut away perspective view of one type of referencepath-arm sensor.

FIG. 7 is a partially cut away perspective view of yet a further type ofreference path-arm sensor.

FIG. 8 is a partially cut away perspective view of still another type ofreference path-arm sensor.

FIG. 9 is a partially cut away perspective view of the styli portion ofanother type of reference path-arm sensor.

FIG. 10 is a block diagram of a turntable VNC (vertical noisecompensator) system in accordance with the invention.

FIG. 11 is a cross sectional side view of one type of vertical actuatorusable in a turntable VNC system.

FIG. 12 is a cross sectional side view of a further type of verticalactuator usable in a turntable VNC.

FIG. 13 is a cross sectional side view of yet a further type of verticalactuator usable in a turntable VNC system.

FIG. 14A is a partly cross sectional side view of a turntable employinga vertical actuator.

FIG. 14B is a cut away partly cross sectional side view of a furtherturntable employing a vertical actuator.

FIG. 15A is a cross sectional side veiw of a retrofit type verticalactuator for use with a conventional turntable.

FIG. 15B is a partly cross sectional side view of a further type ofretrofit vertical actuator for use with a conventional turntable.

FIG. 16A is a block diagram of a prior art electrical tone arm dampingarrangement.

FIG. 16B is a block diagram of a prior art tone arm servo arrangement.

FIG. 17 is a block diagram of a tone arm VNC system according to thepresent invention employing direct reference path sensing.

FIG. 18 is a block diagram of a further type of tone arm VNC system inaccordance with the present invention in which indirect reference pathsensing is employed.

FIG. 19 is a block diagram of a further type of tone arm VNC systemaccording to the present invention in which the error signal is includedin a negative feedback loop.

FIG. 20 is a functional block diagram of a cartridge VNC according tothe present invention.

FIG. 21A is a block diagram showing exemplary combinationinterconnections between transducers in a cartridge VNC according to thepresent invention.

FIG. 21B is a block diagram showing an alternative exemplary combinationinterconection between transducers in a cartridge VNC according to thepresent invention.

FIG. 22 is a perspective view of a portion of a cartridge VNC.

FIG. 23 is a perspective view of a portion of a further cartridge VNC.

FIG. 24 is a block diagram showing a pre-amp VNC.

FIG. 25 is a partially block schematic view of a further embodiment ofthe invention employing a cartridge having a spaced apart secondarystylus.

DESCRIPTION OF THE EMBODIMENTS

In all of the embodiments to be described, only the essential inventivefeatures will be shown or discussed in detail Thus, except whereotherwise specified, amplifiers, attenutators, equalizers,differentiators, integrators, feedback loop compensators, gain controlsand the like are used as ordinarily required in electronic technology.Likewise, except as otherwise discussed the detailed design of styli,sensor transducers, actuator transducers, and the mechanical andelectro-mechanical aspects of disc, cartridges, tone arms, drive motors,and the like will not be treated.

REFERENCE PATH

In the several embodiments, the vertical position of an unmodulatedportion of the record (reference path) is effectively sensed at or inclose proximity to the signal pickup means, typically a stylus. Animportant element of the invention is the recognition that closeproximity, high resolution sensing is useful for reducing mold grainnoise. It follows however, that it is necessary for the reference pathto be as unblemished as possible. For example, it should be free ofscratches. Moreover, the groove "horns" or ridges of material at thegroove edges projecting into the land area should preferably be removedduring the disc manufacturing process.

Polishing of the metal mold is a known method for removing groove horns.Another method is shown in FIG. 2, which shows a further reference pathcutting stylus 4 following the groove cutting stylus 6. Thesubstantially flat bottom edge of the reference path stylus not onlyremoves the groove horns but cuts away residual rumble modulations onthe lacquer master 2 and compensates for any vertical rumble introducedby the recording lathe. A perfectly quiet reference surface is therebydefined for use with the reproducing embodiments of the invention. Inone embodiment, (FIG. 6), a pilot groove provides the reference path. Inthis case, the reference path cutting stylus cuts an unmodulated grooveadjacent the signal groove.

REFERENCE PATH INFORMATION SENSING BY DIRECT MEANS

Reference path information sensing (i.e. sensing of the distortedundulating reference path) is a key element of the various embodiments.A generalized representation of reference path sensing is shown in FIGS.3A and 3B. FIG. 3B is described below under the heading "Reference PathSensing by Indirect Means." Referring to FIG. 3A, the reference pathinformation can be obtained directly, by means of a sensor which followsthe signal stylus laterally but is vertically independent. Sensor 8 isattached to a reference plane. In a conventional turntable theattachment will typically be to the tone arm mounting surface. Intheory, the attaching surface can be any suitable reference surface,including a stable surface apart from the turntable itself. A movablemember 11, forming a portion of sensor 8, follows the surfaceundulations of the disc surface. In practice both disc contacting andnon-contacting sensors are usable, as described below in connection withFIGS. 4A and 4B.

In the version shown in FIG. 4A, the sensor section of the arm may bevertically fixed and the vertical displacement sensing transducer maycomprise nonmechanical means to sense the disc surface 9, such as byultrasonic or capacitive means or by a light beam and detector (e.g.light emitting diode and photodiode). A light beam focused preferably atthe point of contact of the stylus, but with a beam diameterencompassing at least one land area, may be angularly directed at thesurface; vertical variations are then manifested as lateral variations,which are sensed by one or more photodetectors. This technique has theadvantage of providing a relatively wideband error signal without anyattendant mechanical resonances. Warp, rumble and mold grain noises atleast up to several hundred Hz can thereby be compensated.

In the example of FIG. 4A, the tone arm 10, which is vertically fixed,but free to move laterally, has a U-shaped end 12 in which a cartridge14 is pivoted on crosswise pin 16. A light source 18 and detectors 20,22 arrangement similar to that of the sensor version of FIG. 5,described hereinafter, generate the sensor signal.

In the simplest mechanical sensing embodiments the reference path stylusis situated on the cartridge in the manner of FIGS. 6-9 and 25,hereinafter described. In this case the stylus is relatively stifflycoupled to the cartridge body and tone arm, which results in arelatively high vertical resonant frequency of the tone arm. The stylusmay be used along for rumble and mold grain noise reduction. It may alsobe used with a tone arm vertical position sensor to provide referencepath information in a turntable VNC or a tone arm VNC.

A further mechanical version of reference path sensing, shown in FIG.4B, employs a separate stylus laterally coupled to, but verticallyindependent of, the signal pickup cartridge. An arm 24, which can bepivoted for lateral (horizontal) movement only, has a first lateralsupport member 26 on which a tone arm section 28 carrying cartridge 14is pivoted at 30. A second lateral support member and pivot enclosedwithin a housing 32 has a shank 34 carrying secondary stylus 36. Atransducer at the pivot within housing 32 functions as a sensor of thesecondary stylus 38 vertical movement. The secondary stylus is arrangedto lift from the record whenever the signal stylus is lifted.Preferably, the stylus 36 is dimensioned to contact the land areasadjacent the signal stylus 38 of cartridge 14.

Secondary styli may be constructed of any of various long wearingmaterials compatible with disc surfaces and resistant to groovingeffects, such as sapphire or diamond. Sensor transducers may be any ofvarious types known in the art, including, but not limited to:electromagnetic, photoelectric, Hall effect, magnetodiode,potentiometric, or variable resistance, capacitance or inductance. Theuntreated output of the transducer may represent position, velocity,acceleration, or force (as with a pressure responding transducer).

The mechanical characteristics of the reference path sensor assembly canbe optimized for the vertical sensing function only. The frequency ofvertical resonance (sensor flexing and mass) should be placedsubstantially above the highest warp frequency, and indeed well into theaudio band, in order to extend the highest frequency of correctionupwards, thereby to reduce audible rumble and mold grain noise. Unlessand external signal delay is employed, the secondary stylus must besituated very close to the primary stylus--e.g. within 1 mm forcorrection to about 50 Hz. Even closer spacing of about 0.1 mm forcorrection to about 500 Hz is preferable for reduction of mid-range moldgrain noise.

The secondary stylus or sensor means may be positioned slightly inadvance of the primary (signal) stylus to generate an anticipatory errorsignal. This is useful for relaxing the gain and phase requirements ofelectro-mechanical servo loops or for ensuring optimal errorcancellation where mechanical or electrical phase shifts are present, asfor example with low pass filtering of the reference path information.

The mechanical design requirements of the cartridge can be relaxed byspacing the secondary stylus or sensor means farther from the primarystylus than stated above. In order to bring the primary and secondarystylus signals into coincidence, a delay means is employed as describedin further detail below. For optimum correction the delay is preferablyvaried to account for the varying stylus velocity from the outside tothe inside of the phonograph disc. In a low cost design, a fixedcompromise delay may be employed.

REFERENCE PATH-ARM INFORMATION SENSING

Other embodiments of the invention employ reference path-arminformation; this is the signal obtained by sensing the distance betweenthe reference path and arm (i.e. cartridge). This signal willnecessarily include tone arm movements and arm/cartridge resonanceeffects. A first sensor version simply employs the vertical componentinformation from the pickup cartridge, as is known in the prior art.This method provides useful information above the frequency ofarm/cartridge resonance, but is limited to cases and to the frequencyrange in which channel separation is deliberately reduced during disccutting (e.g. below 100 Hz).

In order to obtain reference path-arm information up to higherfrequencies it is necessary to provide a land sensor which isindependent of the signal stylus. Non-mechanical sensing means such asthose mentioned previously in connection with FIG. 4A may be used,however being fixed to the cartridge holding arm or cartridge ratherthan to a vertically fixed arm. An example of such a sensor is shown inFIG. 5. A stereophonic pickup cartridge 40 has a conventionalcantilevered shank 42 and stylus tip 44 shown in engagement with aphonograph disc 9. A light source 46, such as a light emitting diode(LED) or diode laser, for example, generates a beam of light to cause anarea of the record in the order of a millimeters in diameter, orsmaller, to be illuminated. The reflected light is received at one ormore photo receptors 48 and 50, such as photo diodes, in the same manneras that of the description of FIG. 4A. The light illumination locationand diameter are preferably chosen to illuminate the area in which thestylus tip 44 is located and the adjacent land areas so that thereflected light is responsive primarily to local variations in the landat or just preceding the stylus, which variations are representative ofthe rumble and mold grain noise at that point. The output of receptors48 and 50 may be fed to a differential amplifier to provide anindication of local land variations; a suitable circuit arrangement maybe made responsive only to vertical land position variations and not tothe total light reflected, which will depend upon groove modulations.Such techniques are used in automatic slide focusing mechansims, forexample.

Examples of mechanical versions of reference path-arm sensors are shownin FIGS. 6-9 and 25. In each of the embodiments, a dual stylus pickupcartridge is provided in which a conventional stylus tracks the grooveinformation content and the secondary stylus senses warp and rumbleinformation. In the cartridge VNC embodiment of the invention, such adual contact combination has the potential of improving both warp andrumble performance within a unitary, self-contained pickup cartridge.The embodiment of FIG. 25 is not applicable to cartridge VNC's becauseof the signal delay required to compensate for the spaced secondarystylus. A third cartridge to record surface contact device mayoptionally be used, such as a brush or damper of the prior art warptracking devices mentioned above.

An ideal reference path sensing method is shown in FIG. 6, in which ashallow unmodulated pilot groove 52 is provided adjacent the maininformation carrying groove 54 in a phonograph disc 9a. The tip 56 ofsecondary stylus 58 of cartridge 60 rides in the pilot groove and sensesboth vertical and lateral warp and rumble frequencies. The embodimentsof the invention are then adapted to employ both the vertical andlateral information provided. Fortunately, lateral warp and rumble arenot serious problems and it is sufficient in a practical system to dealwith vertical components only.

Referring to FIG. 7, in an arrangement suited to conventional commercialphonograph records, the cartridge 62 has a main stylus having a shank 42and a stylus tip 44 tracking an information carrying groove 54 of aphonograph record 9. The secondary shank 64 and stylus tip 66 aresituated on one or both sides of the main stylus and may fully orpartially encircle it. The contact area may be biased towards theoutsides of the record, if desired, so that pre-echo effects areminimized in the reference path information. The secondary stylus tiphas a substantially flat bottom with operative dimensions sufficientlylarge (e.g. some fraction of a millimeter) so that it rides reliably onat least one land area and is thereby substantially unresponsive tolateral information and to any information content of the groove,responding only to the land height variations which are a measure of thewarp and rumble. The tip 66 is guided laterally by the main stylus, andmay be held in place by a compliant coupling 68 which generallymaintains the relative positions of the two styli but does not interferewith the stylus movements.

Another view of the styli is shown in FIG. 8. An elongated block shapedtip 70 for secondary stylus 72 spanning the land between several groovesis located ahead of and to the outside of the main stylus 42. Asmentioned previously, the secondary stylus tip may optionally have aU-shape (shown by way of example as element 74 in FIG. 9) or an O-shape,surrounding the main stylus. In FIG. 8, the contact area is shown biasedtowards the outside of the record so as to decrease pre-echo effects. Afurther practical matter is that the stylus arrangement shouldpreferably not trap dust but should deflect it away.

In order to relax the mechanical design requirements for the cartridgeof FIGS. 6-9 the secondary stylus may be spaced away from the primarystylus. Preferably, the secondary stylus is located in advance of theprimary stylus in the sense of the phonograph disc rotation so that theprimary and secondary styli signals can be made substantially coincidentby appropriately delaying the secondary stylus signal. Thus the primarystylus may remain in its conventional location and its signal need notpass through a delay line which, if simple, might degrade the signal. Ifa suitable quality delay line is provided the primary stylus may belocated in advance of the secondary stylus and the primary signaldelayed.

FIG. 25 shows an exemplary embodiment in which a cartridge 60, held bytone arm 76 (shown cut-away), has a primary stylus, having a shank 42and tip 44, and a secondary stylus, having a shank 42a and top 44a. Thedisc 9 direction of rotation is shown by the arrow. the secondary stylessignal schematically applied to a delay circuit 280 which preferablyreceives as a variable delay control signal, a signal inversely relatedto the groove velocity. At 33 rpm, the disc velocity at the outside ofthe record is approximately 20 ips; for the inside, about 10 ips. Thus,a greater delay is required when the cartridge is tracing the innergrooves of the disc. A simple way to generate a signal inversely relatedto groove velocity is to derive a signal from the horizontal pickup armposition. Many techniques are well-known for generating a signaldependent on tone arm position. Also, variable delay circuits arewell-known and particularly at audio frequencies are easily implemented.For example, a bucket brigade circuit with an oscillator frequencymodulated by the tone arm position may be used. In the simplest case, acompromise fixed delay time may be used. The compromise value may beselected to favor the outer regions of the disc where pressing anomaliesare greatest. For a stylus to stylus spacing of 0.1 inches, as anexample, a delay of about 0.05 sec. would be nearly optimum for theoutside of the disc and about 0.1 sec. for the inside.

Two suitable techniques for generating a signal indicative of tone armposition are disclosed in U.S. Pat. No. 3,937,476 to Sakai, which isincorporated herein by reference in its entirety. For example, in FIGS.1 and 2 of Sakai, the tone arm position varies the capacitance of avariable capacitor which in turn controls the frequency of anoscillator. For the purposes of the present invention the discriminatedand low pass filtered signal at the output of filter 14 is a suitabletone arm position signal. Note that the low pass filter would remove anyhigh frequency components resulting from record eccentricity. In FIGS.5-7 of Sakai a photoelectric arm position detector is disclosed. Thesignal from Sakai's converter 34 is suitable after low pass filtering bya filter such as filter 14 of Sakai's FIG. 2. The shape of Sakai'sshutter 30 is preferably altered to linearize the arm position signal sothat it follows Sakai's FIGS. 3D and 4D.

The technique of FIG. 25 is applicable to the cartridges and styliconfiguration of FIGS. 6-9, 22 and 23. It is also applicable toturntable, tone arm and pre-amp VNC's. It is applicable to the indirectsensing embodiment of FIG. 3B and FIG. 5 and to the direct sensingembodiments of FIGS. 4A and 4B. In each instance, the technique of FIG.25 permits wider separation of the stylus and sensing area so that therequirements for physical closeness of the stylus and sensor means arerelaxed.

For most tone arm and cartridge configurations, the design of thereference path-arm information sensors should be such that the trackingforce of the main stylus should preferably comprise the main portion ofthe overall cartridge tracking forc in order to avoid affecting the sidethrust forces on the cartridge and to avoid reducing the main stylusforce available for tracking warps and large amplitude signals. Asecondary stylus tracking force a small fraction that of the mainstylus--e.g. 1/4, 1/10, or even less--is adequate to sense therelatively low amplitude and low frequency rumble components. Thesecondary stylus is preferably compliantly connected to the cartridgebody, the compliance preferably being substantially greater than that ofthe main stylus. The above compliance and tracking force considerationsapply primarily to offset tone arm systems in which warp is notcompensated. (In some systems the secondary stylus shank may relativelystiffly coupled the stylus to the cartridge body.) Further, theeffective mass of the secondary stylus and related moving parts,together with the flexing or stiffness properties of the shank, shouldproduce a high frequency resonance well above the highest mold of grainnoise components of interest; thus a resonant frequency of at least 1-2kHz would be suitable for the reference path sensor. As with the designof conventional signal cartridges, suitable mechanical damping can beapplied to the secondary stylus. A low pass mechanical filter may beincorporated if desired, so that the information provided by thesecondary stylus is band limited for reduced sensitivity to dust andsurface scratches.

Information from the reference path-arm sensor is used in one way oranother to cancel corresponding vertical information from the signalstylus. Wholly mechanical cancellation arrangements may be used, ashereinafter described. In the simplest arrangement, with a non-compliantsecondary stylus shank, the vertical movements of the cartridge bodysubtract from the corresponding movements of the primary stylus.Alternatively, secondary stylus information may interact in the magneticor electromechanical arrangements of the main signal transducer in sucha way as to cancel error information. In some arrangements, a separateor coordinated transducer may be provided for the secondary stylus. Thecombination of signals, as by interconnected coils, may be accomplishedwithin the cartridge itself or the signals may be brought out forexternal combination. The signals may be used internally and alsobrought out for use in other embodiments of the invention. For example,the high frequency components from the reference path sensor may beutilized internally or brought out to the preamplifier for mold grainnoise reduction (cartridge VNC ro pre-amp VNC), and the low frequencycomponents may be brought out for dealing with warp via actuation of theturntable or tone arm (turntable VNC or tone arm VNC).

REFERENCE PATH INFORMATION SENSING BY INDIRECT MEANS

As discussed previously, accurate reference path information sensing maybe achieved directly, by means of a vertically fixed sensor. In anapproximation, the vertical position of the tone arm may be used, by themethods known in the prior art. A transducer of the types previouslymentioned in connection with FIG. 4B is mounted between the arm andvertical pivot so as to give an output related to the vertical positionor angle of the tone arm and cartridge. This method of sensing is usefulfor providing reference path information below the arm-cartridgeresonant frequency (e.g. 10 Hz). It is possible to employ a relativelystiffly mounted secondary stylus to raise the resonant frequency of thetone arm. The refernece path-arm sensors described previously inconnection with FIGS. 5-9 and 25 are primarily useful above the resonantfrequency. At or near the resonant frequency, phase and amplitude errorswill be introduced into the reference path information by both of thesemethods. Hence, these methods are useful only at warp or rumblefrequencies somewhat removed from the arm-cartridge resonantfrequency--that is, normally below about 5 Hz and above about 20 Hz.

In an improved method, a substantially error-free warp and rumblesensing signal can be derived, enabling the corrective action to beapplied over the whole frequency range of interest (e.g. 0.5 Hz up toseveral hundred Hz) without interference from arm/cartridge resonance.The method employs a combination of the first and second sensing methodsdescribed in the previous paragraph above (i.e., tone arm sensing andreference path-arm sensing) in a manner shown schematically in FIG. 3B.This approach is based on the recognition the both signals contain thesame error signals (from arm-cartridge interaction effects), but incomplementary form, whereby they can be cancelled to leave a differencesignal accurately indicative of the warp and rumble. The vertical armposition signal x is an indication of the arm 76 to reference planedistance, whereas the reference path-arm signal y is an indication ofthe cartridge 78 to disc distance; the difference z is the warp andrumble amplitude; that is, z=x-y. The arm/cartridge resonance errorsignals contained in x and y signals from the tone arm sensor 80 andreference path-arm sensor 82 are cancelled in combiner 84, providing asubstantially error free reference path information signal 86. The ysignal can be derived by the various means which have been discussed,using either the signal stylus 78A, in some cases providing usefulinformation up to about 30 Hz, or the secondary stylus 78B, by whichuseful information is obtained up to about several hundred Hz.

TURNTABLE VNC

Referring to FIG. 10, a block diagram is shown of a turntable VNC(vertical noise compensator) embodiment of the invention, in whichvertical position or displacement errors in the vicinity of the pickuptransducer are sensed to control the vertical displacement of the discin a closed loop servomechanism system. In effect, a reference plane forthe disc is set by the bias displacement level Y_(o) applied toadder/substractor 90. The reference plane may optionally by madevariable by means of control 82. An error signal on line 94 is developedby subtracting the amplified reference path information signal on line96 from the bias signal. The error signal is applied to an amplifier 98and a vertical actuator 100 that controls the positioning of the disc 9in a direction substantially normal to the disc surface, at least in theregion of the signal stylus. A signal 102 relating to spurious verticaldisplacement of the disc in the vicinity of the pickup transducer isgenerated by the reference path sensor 104 and applied to the applied106.

For reduction of warp, the feedback system should be effective at leastin the frequency region of about 0.5 Hz through about 10 Hz. The systemmay be AC coupled, DC coupled or a combination thereof. For reduction ofrumble and mold grain noise, the bandwidth of the correction action isextended into the audio range (e.g. up to several hundred Hz). Thecorrection may be wholly mechanical, via the vertical actuator.Alternatively, the higher rumble frequencies (e.g. above 50 Hz) may becompensated via a cartridge VNC embodiment or a pre-amp VNC. If desired,a crossover network can be used to divide the treated frequency rangesappropriately. The overall system can thus make a significantcontribution both to reducing warp and its side effects, and to reducingaudible noise.

The spurious vertical displacement dealt with by the system mayoptionally be exhibited by display means 108. Switch 110 may optionallybe provided to break the loop in order to switch off the correctionaction. Blocks 112 and 114 are described in connection with FIG. 11,below.

Means are required to control the disc position in response to sensedvariations in the vertical disc position in the vicinity of the pickuptransducer. The disc height may be varied uniformly across its surfaceor only in the vicinity of the pickup transducer. The necessary linearor angular motions may be accomplished via the disc, the turntable, thedrive motor, or the whole motorboard assembly.

A representative translational vertical actuator, resembling a wooferloudspeaker with a flat cone, is shown in FIG. 11. A metal platter 116is supported by drive shaft 118 in bearing 120. Platter 116 is providedprimarily for rotational mass, in order to reduce wow and flutter, andmay be reduced in mass or even eliminated in some designs. The uppersurface of platter 116 is conical to receive a light-weight phonographrecord supporting turntable 122, which includes index pin 124. Turntable122 may be partly hollowed out, with a cover member 126, to minimizeweight. One or more "spiders", or annular sets of corrugated material128 and 130, suspend turntable 122 from the platter 116. The corrugatedmaterial may be a light-weight fiber material of the type commonly usedin loudspeaker suspensions. A push rod 132 passing through the center ofdrive shaft 118 riding on ball bearing 134 is driven by a moving coil136 which forms part of a moving coil motor assembly 138, used here as amotion transducer for push rod 132. Assembly 138 includes permanentmagnet 140 and a tubular core 142 suspended from the magnet 140 to carrywinding 136 in proper relation to the magnet. Supply leads 144 and 146are driven by amplifier 98 (FIG. 10). Springs or other such supportmeans may be provided to balance the weight of the turntable assemblyand to vertically center the motor unit 138.

Turntable 122 is preferably of very light-weight material, such asfoamed plastic, to minimize the mass required to be moved by thetransducer 138. Also, the push rod to drive shaft interface friction isreduced as low as practicable by using a nylon or other low frictionbearing, for example. Such bearings may be dispensed with in somedesigns, particularly if an additional spider is used at the bottom ofthe pushrod 132. Other types of mechanical coupling, such as hydraulicand pneumatic, may also be used.

The motor assembly 138 preferably provides a positive displacementoutput for a given electrical signal input; this avoids mechanicalresonances of the system. This type of performance may, for example, beprovided by enclosing the motor unit itself within its own servo loop,preferably including a motor position sensor such as 112 in FIG. 10 (andFIG. 11) and a suitable amplifier 114. Alternatively, if the motor unitsimply provides an essentially undamped force, then it is necessary toprovide an appropriate amount of passive damping; this, however,requires a large amount of drive amplifier power. A more practicalmethod is to ensure that the masses and compliances of the turntableassembly result in a resonant frequency either well below or well abovethe frequency range of interest. The resonant frequency might be placedat about 50 Hz, for example, to provide a well controlled behavior up toabout 20 Hz. Passive mechanical damping using viscous materials may beemployed as required. These considerations also apply to the furtherturntable VNC embodiments to be described. Such passive displacementcontrol methods do not interfere with operation of the record playerwhen the VNC is switched off; this is not the case, however, with thetone arm VNC embodiments to be described in which electronic servos arepreferred so that the arm may be handled manually and can track normallywhen the VNC is switched off.

In an alternative embodiment of the vertical actuator, shown in FIG. 12,the transducer 138 is located in a cylindrical cut out 148 in modifiedplatter 116a. Thus, the transducer 138 rotates along with platter 116a.The push rod is thus eliminated, along with its mass and friction.However, in order to power the motor assembly 138, a pair of slip rings150 or other electrical transmission means is provided.

Alternatives to the vertical translation mechanisms which have beendescribed are directed to controlling the disc height only in thevicinity of the pickup transducer. Such method may controllably rock ortilt the turntable so as to provide vertical movement along the linetraced by the pickup cartridge.

In the example of FIG. 13, a tiltable phonograph supporting turntable122a is spaced above a metal platter 116b. Platter 116b is provided onlyfor rotational mass, in order to reduce wow and flutter if necessary,and may be dispensed with in some designs. Turntable 122a has a downwardconical annular portion 152 that is coupled to platter 116b for rotarymotion but permits rocking or tilting of the turntable. Drive shaft 118afor platter 116b, powered by a suitable rotational drive motor, isseated in bearings 120a. A hollow center is provided in drive shaft 118afor tilt rod 132a that terminates in turntable 122a and index pin 124.Tilt rod 132a is coupled by a rotary joint 154 and rod 156 to a movingcoil motor assembly 138, of the type described above, located to providelateral motion to the bottom end of rod 132a and hence, to tiltturntable 122a.

Alternatively, the entire turntable and drive motor assembly can betilted relative to the tone arm and cartridge, in the manner of theembodiment of FIG. 14A. A turntable 158 driven by a motor 160 throughdrive shaft 162 is supported by sub-base 164 suspended by compressionsprings 166, 168 from a base 170, to which the arm and cartridgeassembly 172 is mounted. A moving coil transducer assembly 138controllably moves one end of the sub-base 164 to tilt the turntablerelative to the arm/cartridge assembly.

In a variation of the arrangement of FIG. 14A a displacement transduceris located so as to move vertically the turntable and drive shaft,possibly including the drive motor. For economy, the displacementtransducer may be made a part of the drive motor. FIG. 14B shows anarrangement in which the entire drive motor and turntable are movedvertically. A corrugated annulus 130a, similar to material 130, suspendsthe motor 160 from an annular support 171 from the motorboard 164 topermit vertical movement.

Further tilting embodiments useful as retrofits for existing turntablestructures are shown in FIGS. 15A and 15B. A conventional turntable164a, driven by shaft 162a, has a tilting surface assembly 174 restingon its top surface. Assembly 174 includes a tilting turntable member 176generally coextensive with the size and shape of the underlying existingturntable 164a and having an extended periphery with downward dependingedges. Turntable 176 has a downward depending conical annular portionthat contacts a cone shaped member 177 that ships over the index pin 179on the underlying turntables 164a and spaces the extending portions ofturntable 176 above the underlying turntable to permit tilting. In orderto provide rotational coupling between the turntables, an annulus 178,of sufficient weight and surface friction to couple securely to theunderlying turntable surface, is coupled to a hollowed out under portionof the turntable 176 by means 180 that are rotationally rigid yetyieldable to rocking motion. Corrugated material such as described abovein connection with FIGS. 11 and 12 is suitable. The downward dependingedges 182 of the upper turntable are metallic so that an electromagnet184 arranged to controllably pull on the edge at one location controlsthe turntable tilt.

In FIG. 15B, an alternative rocking mechanism 183 engages the top of arecord 185, being placed in position after the record is put on theturntable. A linear motor 138, as described in relation to FIG. 13,provides the required rocking action through rotary joint 154.

For simplicity in retrofitting such warp compensation devices, the warpsthemselves may be sensed in an approximate fashion at the record edge,at a position substantially where the pickup cartridge intersects therecord, using a lamp and photocell assembly 192. This sensing approachis most accurate at the record edges where warp is greatest. Othersensing means as described above, can be used if greater accuracy isdesired.

An advantage of a disc reproducing system employing a turntable VNC,particularly of the vertical translation types of FIGS. 11, 12 and 14B,is the possible use of a tone arm essentially fixed vertically andmounted only for lateral movement relative to the record. Thissimplifies reference path sensing, which may be done directly, as inFIGS. 4A and 4B. It also follows that if a pivot is used either foraccommodating any residual vertical motion of the disc or for placingthe stylus on the disc, the pivot can be situated very close to thecartridge without fear of introducing warp wow. If desired, the verticalactuator may be controlled so as to effect engagement and disengagementof the record and the cartridge. The vertical actuator can thuseliminate the need for an automatic lifting mechanism associated withthe tone arm.

The substantial elimination of record warp by turntable VNC reproducersis accompanied by a corresponding reduction in the many problemspreviously mentioned in relation to warp. Moreover, the effectiveexistence of "warp-free" records makes possible the design of tone arms,pickup transducers, and signal styli taking this operating conditioninto account. For example, it will be easier to optimize the design ofan offset tone arm which is required to move in a substantially lateraldirection only. The reduced tracking pressure and maximum excursion ofthe stylus result in a different set of electromechanical parameters forcartridge design. The latter observation applies also to the tone armVNC reproducers to be described below.

TONE ARM VNC

Prior art feedback tone arm systems are laid out in the manner of FIGS.16A and 16B. The tone arm vertical actuator is an electromechanicaltransducer so arranged to apply a force to the tone arm or cartridge ina direction normal to the disc surface, in response to an electricalsignal from the sensor and amplifier. A further motor unit can beemployed to perform similarly on a horizontal basis (or 45°/45°).

In one prior art version shown in FIG. 16A, a tone arm sensor isarranged to monitor the vertical velocity of the tone arm; the negativefeedback loop thereby acts to provide damping for the tone arm. Thedamping depends on loop gain, which must not be so high as to interferewith the tracking of warps, which are tracked passively. The overallresult is that the tone arm/cartridge resonance effects are reduced butthat the feedback loop does not directly enter into tracking of the discsurface.

In other prior art versions, shown in FIG. 16B, the attempt is to employa servo loop to track the warps actively. The distance between the tonearm and disc surface is sensed either by a separate transducer or viathe cartridge output signal. This signal includes the arm/cartridgeresonance characteristics; the uncertain and rapid changes of loop phaseand gain in the region of resonance makes compensation difficult, limitsthe loop gain which can be used, and seriously interferes with theeffectiveness of the servo action. The prior art feedback tone armtechniques are thus only partially effective in dealing with theproblems of record warp.

Improved tone arm systems in accordance with the present invention areshown in FIGS. 17, 18 and 19. FIGS. 17 and 18 utilize the improveddirect and indirect error sensing methods, shown in FIGS. 3A and 3B,respectively, which effectively eliminate the effects of arm/cartridgemechanical resonance from the error signal. The embodiment of FIG. 19employs a double closed loop in such a manner that the sensed referencepath-arm information is caused to be substantially the same as referencepath information, thereby eliminating resonance effects. In some of theembodiments, both vertical and lateral operation (or 45°/45°) arepossible, but for simplicity the discussion will be confined to verticaloperation only.

The embodiment shown in FIG. 17 employs an open loop and direct sensingof the reference path via a transducer 8 which is independent ofvertical movements of the cartridge; this type of sensor is illustratedin FIGS. 4A and 4B. The reference path information is amplified inamplifier 196 and applied to the tone arm (cartridge) vertical actuator89, which may be a moving coil motor unit as in the feedback tone armsof the prior art. The signal polarity and gain are set to provide avertical drive to the cartridge body corresponding to the warp andrumble undulations of the disc surface, a condition which will result inzero, or at least a minimum output from the signal cartridge at least inthe warp frequency range. Depending on the gain setting, the system mayalso undercorrect or overcorrect vertical errors. When the gain isoptimally set by gain control 198, the system will be able to effectcorrection through and above the arm/cartridge resonant frequency regiononly if the arm displacements are positively related to the referencepath information signal. That is, the arm must be damped, eithermechanically or electrically, so as to eliminate the arm/cartridgeresonance and obtain a positive displacement effect. The actuatordamping and/or servo considerations discussed previously in relation tothe turntable VNC embodiments are relevant here. An exemplary servo loop200 around the tonearm vertical actuator is shown at the right-handportion of FIG. 17 and includes a tonearm vertical movement sensor 80(the transducer is of the type described in connection with FIG. 4B,however arranged to sense tonearm vertical movement) amplifier 202,adder/subtractor 204 and actuator drive amplifier 206. The gain andother characteristics of the loop 200 are set to obtain good overallwarp and rumble compensation performance up to, say, 20 Hz, with higherfrequencies of reference path information being tapped off at 208 andutilized more conveniently in a pre-amp VNC, to be described. It shouldbe noted that the actuator servo 200 is isolated and used only to obtaina positive displacement effect in the present invention, so that muchhigher values of loop 200 gain may be employed than in the closed looparrangements of the prior art. The first prior art embodiment mentioneduses the actuator loop only to damp the tone arm and not as part of anoverall servo system. The other prior art loops include thearm/cartridge transfer characteristic, severely limiting the usable gainbefore oscillation.

Switch 212 permits opening of loop 200 to deactivate the system for testor demonstration purposes. Display means 108 allows the reference pathinformation to be observed.

Tone arm VNC embodiments may be AC coupled, DC coupled, or a combinationthereof. A bias control (such as 210 in FIG. 17) can be used to setstylus force and/or to raise and lower the stylus. In an AC-only systemthe tracking force can be mechanically determined, as with conventionaltonearms; a bias control may, however, optionally be employed tooverride the error signal for raising and lowering purposes.

FIG. 18 shows a further open loop tonearm VNC in which a reference pathinformation derivation system of the indirect type shown in FIG. 3B isemployed. The gains of amplifiers 214 and 214a are set to conform to theconditions shown in FIG. 3B, whereby substantially pure reference pathinformation is obtained, unadulterated by arm/cartridge resonanceeffects. The operation of the system is essentially the same as that ofthe open loop system of FIG. 17. Under, over, or optimal correction canbe obtained, depending on the setting of the gain control. As in thesystem of FIG. 17, there are no particular constraints on the gain orother properties of the tonearm actuator servo loop 200. Actuatordamping is required only to ensure effective warp and rumblecompensation, not to prevent oscillations. Thus, the loop 200 gain isset to provide adequate damping or a positive displacement effect of thevertical actuator, in order to yield a good warp and rumble compensationin the frequency range of interest (e.g. up to 20 Hz).

For economy, tonearm sensors 80 and 80a may be the same sensor.Likewise, amplifiers 202 and 214 may be the same amplifier, withattenuation as required to provide appropriate levels (gains) at theinuts of the combining networks 216 and 204.

FIG. 19 shows a version of a tone arm VNC in accordance with theinvention in which the warp and rumble error signal itself is enclosedwithin a negative feedback loop. The loop automatically provides tonearm movements which correspond to those of the disc surface. As in theprevious embodiments, it is essential that the correct error signalshould be employed--i.e. reference path information without interferencefrom arm/cartridge resonance effects.

Consider the operation of the system of FIG. 19 if the outer loop 201were broken at point A. The arm would remain stationary throughout thefrequency range of concern (0.5 Hz-20 Hz), provided that sufficient gainis employed in the vertical actuator servo loop (inner loop 200); thegains of amplifiers 202 and 206, for example, may be set to ensure thiscondition without fear of oscillation, as there are no unusualoscillation provoking elements within the inner loop. Under thiscondition the output of the reference path-arm sensor is pure referencepath information. If the outer loop is then closed at point A the armwill follow the reference path but at an amplitude depending on theoverall outer loop gain. The gain of amplifier 218 may be set to providethe desired factor of reduction in output from the reference path-armsensor, which, it should be noted, provides pure reference pathinformation but at a reduced amplitude. Thus, the invention remedies theproblems of the prior art tone arm feedback systems (FIG. 16B) ineffectively eliminating the troublesome arm/cartridge transfercharacteristics from the feedback loop, whereby the arm is actuated byand follows the reference path.

CARTRIDGE VNC

FIG. 20 shows a functional block diagram of cartridge VNC embodiments ofthe invention. Warp, rumble, and mold grain noise from the main stylus(groove-arm sensor 220) are cancelled from the output signal in one wayor another by arrangement 222, which may, for example, be mechanical,magnetic or electrical in nature, using the error information providedby the reference path-arm sensor 224. This is preferably accomplishedentirely within a unitary dual stylus cartridge assembly. The operatingparameters can thus be fixed and pre-set by the cartridge manufacturer,whereby installation either on an original or replacement basis is asimple matter.

The reference path-arm sensor 224 may be of the types previouslydiscussed and shown in FIGS. 5-9. Two independent sets of transducers226 and 228 (e.g. magnets and coils) may be provided within thecartridge, as illustrated schematically in FIGS. 21A and 21B, which showexemplary combination interconnections. The transducer outputs may befurther divided to accommodate the left and right signals. A low passelectrical or mechanical filter may be provided (e.g. 300 Hz low pass),with external control of the characteristics, if desired, to reduce anynon-rumble or non-mold grain noise components sensed by the secondarystylus, such as might be caused by scratches or rough groove edges.Reference path information terminals, shown in FIGS. 20 and 21 may beprovided, whereby low frequency components may be utilized by turntableVNC or tone arm VNC embodiments

In a further version, shown by way of example in FIG. 22, the secondarystylus shank 72, instead of coupling to its own transducer, is coupledto the stator components of the signal transducer, so that rumbleinformation sensed by the secondary stylus 70 cancels out rumbleinformation sensed by the primary stylus 44. That is, for rumblefrequencies there is no relative movement between e.g. the iron ormagnet 230 and the coils 232. Various other analogous cancellationarrangements may be employed, using moving coils, moving iron, movingmagnets and the like. Two like armature elements may share the samestator environment, such as two coils, each associated with its stylus,sharing a common magnetic field.

The aforementioned stator components of the signal transducer mayinclude the cartridge body itself. However, for optimum noisecancellation, the mass to be displaced by the secondary stylus should beas low as possible. Thus, the signal stylus and the secondary styluspreferably have independent compliant connections 231 and 233 to thecartridge body. For optimum tracking, the secondary stylus preferablyaccounts for the lesser portion of the overall cartridge tracking forceand the lesser portion of the overall vertical stiffness.

Further mechanically coupled arrangements are possible in which themovements of the secondary stylus cancel corresponding movements of theprincipal stylus. One example is shown in the embodiment of FIG. 23,wherein a U-shaped stylus tip 74 partially surrounds the main stylus tip44. An A-shaped shank 234 connects tip 74 to first and second members274 and 276 that are fixed relative to the cartridge. The main stylusshank 42 is attached to the cross piece of shank 234 at point 278. Thedistal end of shank 42 is connected to a conventional magnet or ironpiece 242 which cooperates with coils 244. In operation, secondarystylus 74, riding on more than one land area adjacent the groove engagedby main stylus tip 44, responds to rumble frequency components andcancels movements of magnet 242 due to corresponding responses of themain stylus.

PRE-AMP VNC

The reference path-arm signal may be combined with the main stylussignal in the electronic manner shown in FIG. 24. This embodimentfunction in essentially the same manner as the cartridge VNC describedpreviously and is primarily applicable to sensors of the types shown inFIGS. 4A and 4B, and 5, or to separate transducer versions of FIGS. 6-9and 24. Each sensor has a respective pre-amplifier 244 and 246. The nullcontrol 250 is set for optimum noise cancellation in combiner 248. Ifdesired, electrical filtering, signal delay correctors or otherprocessing may be inserted into either or both signal chains in order tooptimize the noise reduction effect under all operating conditions. Forexample, a transient noise suppressor 252 can be employed to reduce theeffect of "pops" and "clicks" caused by scratches on the record and thelike.

The signal on the disc being reproduced can be encoded in electronicnoise reduction form, for example using high frequency compression inorder to reduce high frequency noise when played back, using highfrequency expansion by means of a system known as "Dolby B" or "DolbyC". Such high frequency noise reduction will be most effectivepsycho-acoustically when combined with low frequency disc noisereduction as provided by the various embodiments of the invention. Thesystem of FIG. 24 can provide for playback of a Dolby B encoded disc byoptionally providing a Dolby B type decoder 256, preferably followingsubtractor 248 or, less desirably, in the signal chain of blocks 220 and244 before subtractor 248.

What is claimed is:
 1. In a system for reproducing phonograph recordshaving a movable tone arm carrying a cartridge and pickup stylus, havingan arm-cartridge resonant frequency, for tracking the record groove,apparatus for deriving reference path information as to the verticallocation of an unmodulated surface of the record in the vicinity of thepickup stylus comprisingmeans for sensing the vertical location of themovable tone arm to provide tone arm information which includesarm-cartridge resonant effects, means for sensing the support arm tounmodulated record surface distance in the vicinity of the stylus toprovide refernece path-arm information which includes arm-cartridgeresonant effects, means for time delaying said reference path-arminformation, and means for combining said tone arm information andreference path-arm information out of phase to provide reference pathinformation wherein arm-cartridge resonant effects are substantiallyreduced.
 2. The combination of claim 1 wherein the support arm tounmodulated record surface distance is sensed more than about 1millimeter from the stylus location.
 3. The combination of claim 2wherein said delay means is variable and includes means for generating acontrol signal inversely related to the styli groove velocity.
 4. Thecombination of claims 1 or 2 wherein the support arm to unmodulatedrecord surface is sensed with respect to the record surface land areaadjacent the record groove location of said stylus.
 5. A system forreading out modulated groove portions and unmodulated portions of aphonograph record, said system comprising:pickup means responsive tomodulated portion of the record for generating a first signal component;sensing means for sensing the unmodulated portions of the record in thevicinity of the modulated portions read by the pickup means forgenerating a second signal component; and delay means for delaying oneof the first and second signal components relative to the other so thatthe two signal components coincide substantially in time.
 6. The systemof claim 5, wherein the pickup means and the sensing means are spacedapart and the time delay introduced by the delay means compensates forthe time difference between the generation of the two signal componentscaused by the spatial separation between the pickup and sensing means.7. The system of claim 5, wherein said delay means is variable, saidsystem further including means for generating a control signal inverselyrelated to the speed at which the record is read by the pickup andsensing means for controlling the time delay introduced by the delaymeans.
 8. The system of claim 5, further comprising means responsive tothe undelayed signal component and the delayed signal component forsubstantially cancelling one component from the other.
 9. The system ofclaim 8, further comprising a housing for holding the pickup means andthe sensing means wherein said cancelling means is located external tosaid housing.
 10. The system of claim 5, further comprising a housingfor holding the pickup means and the sensing means wherein said firstand second signal components are provided externally to said housing forfurther processing.
 11. The system of claim 5, wherein the pickup meanscomprises a first stylus and the sensing means comprises a secondstylus.
 12. The system of claim 11, wherein the second stylus engagesthe record greater than a distance in the order of 1 mm from said firststylus.
 13. The system of claim 11, wherein the tracking force of saidsecond stylus is a small fractional part of that of the first stylus.14. The system of claim 11, wherein said second stylus is arranged toread the record in advance of said first stylus.
 15. The system of claim11, wherein said second stylus is offset from said frist stylus toreduce the sensing of pre-echo.
 16. The system of claim 11, wherein saiddelay means is variable, said system further including means forgenerating a control signal inversely related to the styli groovevelocity for controlling the time delay introduced by the delay means.17. A phonograph disc playback apparatus comprising:a turntable, adaptedto support a disc thereon; pickup means responsive to modulated portionsof the disc for generating a first signal component; sensor means forsensing the vertical deviation of the disc on the turntable in thevicinity of the pickup means to provide a second signal component; delaymeans for delaying one of the first and second signal componentsrelative to the other so that the two signal components substantiallycoincide in time; and an actuator responsive to a signal originatingfrom the sensor means for moving the portion of the disc at the pickupmeans to reduce vertical deviation of the disk at the pickup means. 18.The apparatus of claim 17 wherein the sensor means senses the discgreater than a distance in the order of 1 mm. from the pickup means andwherein the time delay introduced by the delay means compensates for thetime difference between the gneration of the two signal componentscaused by the spatial separation between the pickup means and the sensormeans.
 19. The apparatus of claim 17 wherein said delay means isvariable, said apparatus further including means for generating acontrol signal inversely related to the speed at which the disc isplayed back for controlling the time delay introduced by the delaymeans.
 20. A phonograph disc playback system comprising:a cartridge; astylus carried by the cartridge for providing an audio signalrepresenting the groove modulation of a disc; means carried by thecartridge for sensing vertical deviations of an unmodulated part of thedisc in the vicinity of the stylus to provide a vertical deviationsignal, said sensing means being substantially more compliant than thestylus; and means for delaying one of the audio and the verticaldeviation signals relative to the other so that the two signalssubstantially coincide in time.
 21. The system of claim 20 furthercomprising a compensating electrical coupling between said stylus andsensing means wherein the sensing means at least partially removes fromthe audio signal the effects of vertical deviations sensed by thesensing means.
 22. The system of claim 20 wherein the unmodulated partof the disc sensed by the sensing means is at a distance greater thanabout 1 mm. from the stylus.
 23. The system of claim 20 wherein saiddelay means is variable, said system further including means forgenerating a control signal inversely related to the speed at which thedisc is played back for controlling the time delay introduced by thedelay means.
 24. A phonograph disc playback apparatus comprising:acartridge; a first sensor for sensing vertical deviation of thecartridge to provide a first signal component; a second sensor forsensing vertical deviation of the disc relative to the cartridge toprovide a second signal component; delay means for delaying one of thefirst and second signal components relative to the other so that the twosignal components substantially coincide in time; and an actuatorresponsive to an error signal which is a function of the differencebetween the undelayed signal component and the delayed signal component,said actuator being adapted to move the cartridge in response to theerror signal, said error signal being representative of the verticaldeviation of the disc relative to a reference plane.
 25. The apparatusof claim 24 wherein the second sensor senses the disc at greater than adistance in the order of 1 mm. from the first sensor and wherein thetime delay introduced by the delay means compensates for the timedifference between the generation of the two signal components caused bythe spatial separation between the two sensors.
 26. The apparatus ofclaim 24 wherein said delay means is variable, said apparatus furtherincluding means for generating a control signal inversely related to thespeed at which the record is played back for controlling the time delayintroduced by the delay means.
 27. In a system for reproducingmechanical recordings having corresponding adjacent modulated andunmodulated portion, the system including a pickup means, said systemcomprising:means including a movable member attached to a referencesurface and independent of said pickup means for contacting anunmodulated portion of the record adjacent a corresponding modulatedportion being read by the pickup means, means associated with saidmovable member for sensing the position of said movable member toprovide vertical location information, and delay means for delaying saidvertical location information, wherein said delay means in variable andincludes means for sensing the lateral position of the pickup means withrespect to the record to vary said delay.
 28. The combination of claim27 wherein said system is for reproducing phonograph recordings, saidmodulated portion are grooves, said unmodulated portions are the landadjacent grooves, and wherein said movable member contacts the land. 29.The combination of claim 28, wherein said pickup means includes acartridge and stylus for tracking the record grooves, said membercontacting the land adjacent the stylus.
 30. The combination of claim 29wherein said member contacts the land more than about 1 millimeter fromthe stylus location.